Recent years have seen an emergence of new and improved tubular metallic softball and baseball bats. The most common tubular bat is the aluminum single-wall tubular bat. Such bats have the advantage of a generally good impact response, meaning that the bat effectively transfers power to a batted ball. This effective power transfer results in ball players achieving good “slugging” distances with batted balls. An additional advantage of such aluminum bats is the improved durability over crack-prone wooden bats.
Despite the advantages of tubular aluminum bats, there is an ongoing effort to improve the performance and durability of the conventional design. Generally speaking, bat performance is a function of the weight of the bat, the size of the hitting area or “sweet spot” of the bat, and the impact response of the bat. The durability of a bat relates, at least in part, to its ability to resist denting and depends on the strength and stiffness of the tubular frame. While recent innovations in bat technology have increased performance and durability, most new bat designs typically improve performance or durability at the expense of the other because of competing design factors. For example, an attempt to increase the durability of the bat often produces an adverse effect on the bat's performance.
More specifically, the impact response of a bat depends on the bat wall's elasticity, rebound recovery time, and rebounding force. Generally, impact response is optimized when the bat undergoes maximum elastic deflection and then rebounds with the greatest force in the shortest amount of time. The elasticity of a bat can be increased by reducing the thickness of the bat's tubular frame. In contrast, the durability of a bat generally is improved by increasing the thickness of the tubular frame. Consequently, a bat having a relatively thin tubular wall is capable of large elastic deflection, but may be vulnerable to undesirable local plastic deformation (or “denting”). On the other hand, a relatively thick tubular wall is more durable but may be too stiff to achieve optimum slugging performance. Thus, enhancing one design aspect of a bat often compromises another.
Another example of competing design factors concerns the bat's optimum hitting area or “sweet spot.” The sweet spot is typically located near the center of the impact area of the bat. The performance of the bat drops off considerably when a ball impacts the bat outside the sweet spot, for example, near the end of the bat. When this occurs, the batter feels greater vibrations and transfers less energy from the bat to the ball. An obvious way to increase the sweet spot of a bat is to increase the length and circumference of the bat. This option is constrained by institutional rules and regulations. In addition, an increase in the overall size of the bat undesirably adds weight, often causing reduced bat speed and less slugging distance. (A hitter often can increase bat speed by using a lighter bat, thereby increasing the force transferred to the ball upon impact.
An example of a bat incorporating a composite insert is shown in U.S. Pat. No. 5,364,095. This patent discloses a tubular aluminum bat having a carbon composite insert to increase the “stiffness” of the metal tube. The insert is made of multiple fiber layers, each layer having bidirectional woven fibers directed at 0 and 90 degrees relative to the axis of the bat. The insert is bonded to the barrel portion of the surrounding metal tube or frame and presses outwardly on the frame to produce a pre-load stress of several thousand pounds per square inch. The insert appears to be formed from multiple layers of glass and carbon fiber material (thickness of 0.03 to 0.05 inch) so as to be a self-supporting structure capable of withstanding several thousand pounds of compressive stress. This design gives the bat a relatively stiff, rigid tubular frame which appears to be capable of limited elastic deformation, a less than ideal trait if the goal is to optimize slugging performance. (One would expect this design to behave like a single-wall bat in which the compressive stress must be overcome before the wall begins to deflect.)
While composite materials offer the advantage of a high strength to weight ratio, such materials also present design challenges. Composite inserts and bat frames are prone to wear and tear due to the inter-laminar shear which can occur between bonded layers of composite material. The deflection caused when a ball impacts the bat produces shearing stresses between the composite layers, sometimes causing the bond between adjacent layers to fracture or separate (especially over time).
Additionally, the composite materials are typically formed as sheets, which are wrapped into a generally cylindrical shape. These sheets typically have seams formed where two wrapped edges of the sheet meet. The seam typically extends the length of the sheet in a position that is substantially parallel with the longitudinal axis of the insert or the bat frame. Multilayered composite inserts utilize two or more sheets, each having a separate seam. Often the longitudinally extending seams of two or more sheets will generally overlap each other. These longitudinally extending seams can be subjected to large impact loads, particularly when the seam or seams align with the line of action of contact between the ball and the bat, commonly referred to as the “line of action” of the bat. The line of action of the bat also refers to the longitudinal portion of the bat, which upon impact with a ball, receives an impact load and transmits the load longitudinally to the handle of the bat. It is not uncommon for bats having a composite layer and a longitudinally extending seam to crack, separate, or otherwise fail at a point along the seam. Further, a bat including at least one composite layer having a longitudinally extending seam, can have inconsistent or varied performance characteristics depending upon the orientation of the bat, and in particular the location of the seam of the composite layer, in relation to the location of impact with the ball. The slugging performance of such a bat when impacted by a ball along the composite layer's longitudinal seam will be lower than when a ball contacts the bat at a location away from the longitudinal seam.
Thus, despite the advantages offered by composite materials, there are a number of drawbacks associated with using such materials including the potential for reduced elastic deflection, a tendency of the composite layers to separate over time due to inter-laminar shear, the susceptibility of the composite insert to fail along the longitudinal seam of the insert, and inconsistent slugging performance resulting from a longitudinal seam of a composite layer of a bat.
As a result, there is a need for a tubular bat that offers at least some of the advantages of composite materials without the constraints. There is a need for a tubular bat that provides excellent slugging performance and improved durability. There also is a need for a multi-wall bat which has a relatively thin barrel wall and yet exhibits excellent durability. Further, there is a need for a single wall bat having the excellent durability characteristic of most single wall bats as well as improved slugging performance. It would be advantageous to provide a bat including an insert having at least one composite layer with an improved seam orientation that is less susceptible to failure and therefore provides improved reliability. What is needed is a bat having an insert with at least one layer of composite material that provides the bat with consistent slugging performance.